Crude oils, and middle distillates obtained by distillation of crude oils, such as gas oil, diesel oil, or fuel oil, contain various amounts of long-chain paraffins (waxes) depending on their origin. At low temperatures, these paraffins separate out as tabular crystals, some with inclusion of oil. This has a substantial adverse effect on the flow of the oils and the distillates obtained from them. Solid deposits occur and frequently lead to problems in the production, transportation, and use of mineral oil products. Thus, in the cold season, blockage of the filters occurs, for example in diesel engines and furnaces, which prevents reliable metering of the fuels and finally results in complete interruption of the motor fuel or heating agent feed. The transportation of middle distillates through pipelines over large distances in winter can also be adversely affected by the precipitation of paraffin crystals.
It is known that undesirable crystal growth can be inhibited by suitable additives and hence an increase in the viscosity of the oils prevented. These additives, known as pour point depressants or flow improvers, achieve these results of changing the size and shape of the wax crystals formed. The flow behavior and low-temperature behavior of mineral oils and mineral oil distillates are described by the pour point (according to DIN 51 597) and the cold filter plugging point (CFPP, according to DIN 51 428). Both parameters are measured in .degree.C.
Typical flow improvers for crude oil and middle distillates are copolymers of ethylene with carboxylates of vinyl alcohol. Thus, according to DE 11 47 799 B1, oil soluble copolymers of ethylene and vinyl acetate having molecular weights between about 1,000 and 3,000 are added to motor fuels or heating fuels derived from mineral oil distillates having a boiling range between about 120.degree. and about 400.degree. C. Copolymers which contain about 60% to about 99% by weight of ethylene and about 1% to about 40% by weight of vinyl acetate have proven particularly suitable.
It is also known that copolymers of the stated qualitative composition which have been prepared by free radical polymerization in an inert solvent at temperatures of about 70.degree. to about 130.degree. C. and pressures of 35 to 2,100 atmospheres gauge are particularly effective (DE 19 14 756 B2).
DE 21 02 469 C2 relates to the use of ethylene copolymers having a molecular weight of 500 to 4,500 and a comonomer content of 30% to 75% by weight. They are obtained by polymerizing ethylene with vinyl esters of 4 to 10 carbon atoms, and with acrylates and methacrylates which are derived from alcohols having 1 to 12 carbon atoms, together in the absence of solvents and emulsifiers.
The efficiency of the known additives for improving the properties of mineral oil fractions is dependent on their composition and hence on the origin of the mineral oil from which they have been obtained. Additives which are outstandingly suitable for favorably influencing the properties of fractions of one crude oil may lead to completely unsatisfactory results in distillates of crude oils of a different origin.
For economic reasons, it is of interest to increase the yield of middle distillate heating fuels and motor fuels. One method for achieving this is the addition of relatively large amounts of heavy gas oil thereto. Another possibility for improving the middle distillate yield is to widen the boiling range, i.e. to separate off middle distillate fractions whose final boiling point is above 370.degree. C. Both methods result in an increase in the higher molecular weight n-paraffin content. The known additives, such as copolymers of ethylene and vinyl acetate, provide only an unsatisfactory improvement in flow in middle distillate fractions having a final boiling point above 370.degree. C.